docs: def nsummary(self,prn=None, lfilter=None):
          """prints a summary of each packet with the packet's number
  prn:     function to apply to each packet instead of lambda x:x.summary()
  lfilter: truth function to apply to each packet to decide whether it will be displayed"""
          for i, p in enumerate(self.res):
              if lfilter is not None:
                  if not lfilter(p):
                      continue
              print(conf.color_theme.id(i,fmt="%04i"), end = " ")
              if prn is None:
                  print(self._elt2sum(p))
              else:
                  print(prn(p))